1. Field of the Invention
The present invention relates to a pneumatic tire that achieves reduction in road noise, in particular, the noise inside a vehicle produced due to tire vibration when the vehicle drives on a rough road surface, without degrading steering stability.
2. Description of the Background Art
Conventionally, methods of reducing the noise inside a vehicle produced due to tire vibration when the vehicle drives on a rough road surface, i.e., road noise, are attempted.
For instance, there has been suggested a method of reducing tire vibration in which a tread portion is formed as a two-layer structure of cap rubber and base rubber, and at the same time, a rubber composition having large hysteresis loss or rubber having low modulus of elasticity is utilized for cap rubber to reduce the rigidity in the tread portion, or a method of reducing tire vibration in which a thickness of the tread portion is increased while the rigidity in a bead portion is reduced so as to reduce the spring constant of the overall tire. Such methods, however, would degrade steering stability due to reduction in tire rigidity, and thus are not practical solutions.
On the other hand, in Japanese Patent Laying-Open No. 2-60803, a method is proposed in which low hardness vulcanized rubber having a carbon black content of at most 20 parts by weight in the form of particles having a mean particle diameter of 100 to 1000 xcexcm is evenly dispersed in the cap rubber, in a volume fraction of 3% to 10% in relation to the entire cap rubber of a two-layered tread portion. Even with such method, however, no tire can be produced that achieves both road noise reduction and good steering stability at the same time.
The object of the present invention is to provide a pneumatic tire that achieves reduction in road noise due to tire vibration when a vehicle drives on a rough road surface, without degrading steering stability.
The present invention is a pneumatic tire characterized in that a tread portion is formed by at least two layers consisting of cap rubber on a road-contact surface side and base rubber on a side adjacent to a belt layer, where hollow particles having a mean particle diameter of at most 500 xcexcm are mixed in a volume ratio of 2% to 40% into said base rubber, and a thickness of the base rubber is made to be at least 1.0 mm.
Moreover, the hollow particles preferably have strength of a rate of break down of at most 40% at a hydrostatic pressure of 500 kg/cm2.
Further, a thickness of the base rubber is preferably in a range of 1.0 mm to 5.0 mm.